Protection devices can be used for protecting electrical components in electronic systems.
Traditionally, mechanical fuses have been used as electrical safety devices to provide protection for downstream circuitry against overcurrent events in electrical circuits. In a mechanical fuse, in the event of an overcurrent event such as a short circuit or overload, a metal wire or metal strip within the fuse device melts and current flowing in the circuit is interrupted. In this manner, the electrical component is not subjected to excess current over a period of time that could cause it to be irreversibly damaged. Depending upon the specific application, a mechanical fuse can be designed to accommodate specific current and voltage ratings, breaking capacity and response time. However, mechanical fuses are sacrificial circuit elements and, once an overcurrent event occurs, must be replaced. Further, the overcurrent level must exceed the current rating of the fuse by an amount that is subject to variation due to manufacturing tolerances, and the response time of the fuse may range from milliseconds to seconds, also due to manufacturing tolerances.
An alternative to mechanical fuses is the circuit breaker, which is an electrical switch that automatically interrupts current flowing in a circuit when an overcurrent event occurs, but which can be reset after the overcurrent event rather than needing to be replaced. However, circuit breakers are usually large devices that are expensive and can be slow, and so they are not suitable for all applications.
Another alternative to mechanical fuses are positive temperature coefficient (PTC) devices, which are electrical components whose resistance increases with increasing temperatures. Examples of PTC devices include resistors and thermistors. However, whilst PTC devices are relatively inexpensive, a disadvantage of using PTC devices as a protection device is that they are slow acting, sensitive to ambient temperature and can dissipate a lot of power.
The mechanical fuse, circuit breaker and PTC device examples that are described above are all discrete protection devices. In recent times, another alternative to mechanical fuses that has started to gain popularity is the electronic fuse (eFuse). Unlike, the mechanical fuse, circuit breaker and PTC devices, the eFuse may be a discrete component or an integrated circuit component that is used to limit current and sometimes voltages during fault conditions. Amongst its many advantages, the eFuse has a low and accurate current rating, a fast response time and is resettable. However, eFuses are sometimes unable to provide adequate protection for applications that require high currents, for example, downstream circuitry that operate at >10 A.